1. Field of the Invention
The invention is concerned with techniques for forming mineral fibers from a drawable material and in particular from a molten material with a high melting point, for example a material of the basalt or blast furnace slag type. More precisely, the invention relates to an improvement in the so-called free centrifugation fibring techniques in which the material to be fibered is brought to the molten state at the periphery of centrifuging wheels and is entrained by these wheels so that a part of the material becomes detached therefrom and is transformed into fibers under the effect of centrifugal force, the remaining non-transformed part being sent back to another wheel or, after the last wheel, falls down in the form of granules.
2. Background of the Related Art
In order to employ the fibring technique briefly outlined hereinabove, generally three or four wheels are used which are disposed one beside another, two successive wheels in the path of the molten material rotating in opposite directions. The first wheel is supplied with molten material by a spout and mainly serves to accelerate the material which is passed on towards the second wheel and so on until it reaches the last wheel, the flow of material diminishing at each wheel by the amount of the quantity of fibers formed. With such a fibring process--partial in that none of the wheels is sufficient for complete transformation into fibers--it is very important to provide effective means of separating the fibred material from the non-fibred material. Indeed, this latter only helps to increase the weight of the end product and give it a particularly unpleasant feel. Independently of this first problem, the fibred material has to be routed to a receiving means, for example an endless belt fitted with vacuum chambers which transports the fibers to processing devices downstream of the line, such as a lapper, a binder polymerizing oven, etc.
This dual function of separating granules and transporting fibers falls to a current of air introduced at the periphery of the centrifuging wheels, substantially parallel with the axes of rotation of the wheels. Thus, this current of air entrains the fibers in a direction at right-angles to the direction in which they are formed. In Patent Application FR-A-2 322 114, it has been proposed to impart to the current not only an axial velocity but a certain tangential velocity in the direction of movement of the rotor in order to open up the torus consisting of the fibers around the centrifuging wheel; thus, the fibers have a tendency to spread out immediately over a greater volume so that there is a reduced risk of seeing the fibers stick together in tufts. In fact, these tufts would result in a reduced tensile strength in the product. Furthermore, it is known from Patent Application WO-A-8 807 980 to introduce the current of air otherwise than parallel with the axes of rotation of the centrifuging wheels, in fact in a slightly diverging direction, so forming a blowing cone; furthermore, a tangential component of velocity is likewise imparted to the currents of air, the object being to minimize interaction between the gas current and the centrifuging wheels in order to reduce friction between this gas current and the currents of air generated by the high speed rotation of the centrifuging wheels.
These various improvements do not make it possible to obviate the problem caused by the formation of fiber tufts which is encountered in all the relatively calm zones surrounding the centrifuging wheels, particularly the central zone between the wheels. In the end, these tufts are carried away with the flow of newly formed fibers but generally this occurs sufficiently late that the resin which is sprayed onto the fibers while they are being formed is already partly polymerized; consequently, these tufts crop up again in the finished product as isolated rovings which tend to reduce the tensile strength because for practical purposes they are not connected to the rest of the fibers. Furthermore, these rovings entrain with them non-fibred particles which make only a very slight contribution to the heat resistance of the rock wool mat. Above all, their presence penalizes the lightest products, particularly the finished products the density of which is less than 100 kg/m.sup.3. As indicated in the aforesaid Patent Application FR-A-2 322 114, it is not desirable excessively to increase the quantity of air introduced at the periphery of the centrifuging wheels, in order to avoid excessive cooling of the wheels. Consequently, air is not blown into the intermediate space between the wheels and the quantity of air blown outside the wheels is limited, creating calm zones with the disadvantages which have been discussed.